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== Publication Abstract from PubMed ==

The crystal structure of the cyclic octanucleotide d&lt;pATTCATTC&gt; contains two independent molecules that form a novel quadruplex by means of intermolecular Watson-Crick A.T pairs and base stacking. A virtually identical quadruplex composed of G.C pairs was found by earlier x-ray analysis of the linear heptamer d(GCATGCT), when the DNA was looped in the crystal. The close correspondence between these two structures of markedly dissimilar oligonucleotides suggests that they are both examples of a previously unrecognized motif. Their nucleotide sequences have little in common except for two separated 5'-purine-pyrimidine dinucleotides forming the quadruplex, and by implication these so-called "bi-loops" could occur widely in natural DNA. Such structures provide a mechanism for noncovalent linking of polynucleotides in vivo. Their capacity to associate by base stacking, demonstrated in the crystal structure of d(GCATGCT), creates a compact molecular framework made up of four DNA chains within which strand exchange could take place.

The bi-loop, a new general four-stranded DNA motif.,Salisbury SA, Wilson SE, Powell HR, Kennard O, Lubini P, Sheldrick GM, Escaja N, Alazzouzi E, Grandas A, Pedroso E Proc Natl Acad Sci U S A. 1997 May 27;94(11):5515-8. PMID:9159103<ref>PMID:9159103</ref>

From MEDLINE&reg;/PubMed&reg;, a database of the U.S. National Library of Medicine.<br>

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== References ==

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